1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate whereby a printing plate precursor including a switchable polymer is image-wise exposed and treated with a gum solution and the plate is developed and gummed in a single step.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout, and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called a plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided into three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light; and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Laser sources have been increasingly used to expose a printing plate precursor which is sensitized to a corresponding laser wavelength. Typically, an infrared laser diode emitting around 830 nm or a Nd-YAG laser emitting around 1060 nm can be used.
In the thermal plate, the material is exposed to heat or to infrared light and the generated heat triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Most thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The plates are usually processed in an alkaline developer having a pH>10. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Currently, most commercial lithographic plates require an additional gumming process after the exposed plate is developed and before it is put on the press, in order to protect the plate from contamination, e.g., by oxidation, fingerprints, fats, oil or dust, or from damaging, e.g., by scratches during handling of the plate. Especially the non-printing areas defined by the revealed hydrophilic support are very sensitive to contamination or damaging and need to be protected by a gum. Such a wet processing step by an alkaline developer and such an additional gumming step are not convenient for the end-user because it is a time consuming step and requires two wet stations, a processing station and a gumming station.
Thermally switchable polymers have been disclosed for use as an imaging material in processless printing plates wherein no wet processing is required for obtaining the lithographic differentiation between the ink-accepting areas and fountain solution-accepting areas and wherein no gumming is needed to protect the non-printing areas. By “switchable,” it is meant that the polymer, upon exposure to heat, is changed either from hydrophobic to more hydrophilic (or oleophobic) for a negative-working plate or from hydrophilic to more hydrophobic (or oleophilic) for a positive-working plate.
Polymer coatings which undergo a change in surface properties in response to light exposure are known in the art. WO92/09934 discloses coatings that become hydrophilic as a result of irradiation to UV/visible light and that include a photochemical source of a strong acid and an acid-sensitive polymer, derived from a cyclic acetal ester of (meth)acrylic acid such as tetrahydropyranyl(meth)acrylate. There is no disclosure of laser addressability.
EP-A 652 483 discloses a lithographic printing plate requiring no dissolution processing which includes a substrate bearing a heat-sensitive coating including a photothermal converter, which coating becomes relatively more hydrophilic under the action of heat.
EP-A 980 754 discloses a method for making a lithographic printing plate whereby the precursor, including a photothermal converter and a polymer having at least either carboxylic acid or carboxylate groups capable of causing thermal decarboxylation, is exposed by IR-laser.
EP-A 1 084 861 discloses a positive-working imaging member composed of a heat-sensitive surface imageable layer having a heat-sensitive sulphonate polymer and a photothermal conversion material. Upon application of thermal energy, the sulphonate groups decompose rendering exposed areas more hydrophilic. The exposed imaging member can be contacted with a lithographic printing ink and used for printing without post-imaging wet processing.
U.S. Pat. No. 5,985,514 discloses an imaging member composed of a hydrophilic imaging layer, having a hydrophilic heat-sensitive polymer containing heat-activatable thiosulphate groups, and optionally, a photothermal conversion material. Upon application of thermal energy, the polymer is crosslinked and rendered more hydrophobic. The exposed imaging member can be contacted with a lithographic printing ink and a fountain solution and used for printing with or without post-imaging wet processing.
U.S. Pat. No. 6,455,230 discloses a method for making a lithographic printing plate whereby an imaging element is image-wise exposed to a high laser energy. The imaging element includes, on a lithographic hydrophilic support, a heat-sensitive coating including a light-to-heat converting compound and a compound which becomes more hydrophilic under the action of heat.
EP 960 729 discloses a heat-sensitive imaging element for providing a lithographic printing plate which requires no dissolution processing. The imaging element includes a support and as a top layer a heat switchable image forming layer including a light-to-heat converting compound, a hardened binder, and a heat switchable polymer containing aryldiazosulphonate units. The use of compounds or polymers containing aryldiazosulphonate groups in printing plates is also disclosed in EP 507 008, EP 339 393, EP-A 1 267 211, and EP 771 645. In EP-A 1 267 211, the imaging layer includes a polymer having aryldiazosulphonate units and/or aryltriazenylsulphonate units and a compound capable of generating a radical and/or an acid upon exposure to UV light. In EP 771 645, the imaging element, including a polymer containing aryldiazosulphonate units, is image-wise exposed and on-press processed.
In the printing plates of the prior art, the lithographic differentiation between the ink-accepting areas and the fountain solution-accepting areas is formed by a chemical reaction of the switchable polymer upon image-wise exposing and without a dissolution processing. This principle has the disadvantage that the hydrophilic properties of the non-printing areas are insufficiently hydrophilic, whereby fast toning occurs at the non-printing areas. By “toning,” it is understood to mean the tendency of ink-acceptance at non-printing areas during printing.
Another problem for the printing plates in the prior art including a switchable polymer and without a dissolution processing is the lack of a visible image between exposure and processing. Although it is known to add a colorant to the coating, so as to obtain a visible image after removal of the non-printing areas of the coating by the processing, this does not make it possible to distinguish an exposed plate from an unexposed plate immediately after the image-wise exposure, let alone to inspect the image quality after the exposure because the visible image is only revealed after on-press processing or during printing on the press. Moreover, on-press processable plates normally do not contain a colorant because the on-press removal of the non-printing areas of the coating may cause contamination of the fountain solution and/or the ink and it may take an unacceptable number of printed copies before the contamination by the colorant has disappeared.
Another problem associated with on-press processing with fountain solution and ink is an insufficient clean-out of the non-printing areas.
WO 02/101 469 discloses a method of processing an imageable element useful as an alkaline-developable lithographic printing plate precursor wherein the element is developed and gummed with an aqueous alkaline developing-gumming solution including a water-soluble polyhydroxy compound having a specific structure.
EP 1 342 568 discloses a method for making a heat-sensitive lithographic printing plate wherein the image-wise heated precursor, including a coating of hydrophobic thermoplastic polymer particles which coalescence on heating, is developed with a gum solution. A practical preferred embodiment of this type of printing plates was introduced by Agfa under the trade name Azura.
In WO 2005/111727, a method for making a lithographic printing plate is disclosed wherein an image-wise exposed precursor, including a photopolymerizable coating, is developed with a gum solution.